This image shows how some polycrystal grain boundaries feel the heat more than others. Image: Institute of Industrial Science, the University of Tokyo.
This image shows how some polycrystal grain boundaries feel the heat more than others. Image: Institute of Industrial Science, the University of Tokyo.

Polycrystals are solid materials made up of lots of small crystals. The points where these crystals meet are known as grain boundaries. Grain boundaries are important because they can affect the way the polycrystal behaves, but conventional analysis techniques are unable to study grain boundaries in nanoscale detail.

Now, researchers from the University of Tokyo Institute of Industrial Science in Japan have shown that electron energy loss spectroscopy (EELS) can be used to investigate the effect of heating on the grain boundaries of strontium titanate (SrTiO3) in nanoscale detail. They report their findings in a paper in Nano Letters.

Grain boundaries affect the way ions move through a material, the way it conducts and reacts to heat, and the way it responds when forces are applied. They therefore play an import role in deciding whether a material is suited for a particular purpose.

The coefficient of thermal expansion (CTE) indicates how the size of a material changes when it is heated. If this coefficient is different around the grain boundaries compared with the bulk of a material, then cracks can form, which can lead to large-scale failures.

The techniques conventionally used to investigate local thermal expansion do not have the nanoscale resolution required to directly measure expansion around the grain boundaries. The researchers therefore used EELS with scanning electron microscopy to enhance the resolution.

“We looked at the thermal expansion around two different grain boundaries in SrTiO3 – one where the grains met at an angle of 36.8° (which has the particular name S5) and another where they met at 45°,” explains study first author Kunyen Liao. “Specifically, we investigated how the CTE changed perpendicular to these grain boundaries when the temperature was increased over the range 100–700°C.”

The researchers found that the CTE perpendicular to the S5 grain boundary was three times greater than expansion in the bulk, whereas the CTE perpendicular to the 45° grain boundary was only 1.4 times greater. These findings provide directly measured evidence that grain boundaries enhance the CTE, which has important implications for choosing materials for specific applications.

“In addition to revealing the variation in thermodynamic properties at different grain boundaries in SrTiO3, our findings demonstrate the potential of EELS for providing nanoscale detail of local properties,” says corresponding author Teruyasu Mizoguchi. “We hope that our study will provide a means of establishing the local thermal properties of a range of different materials and aid the selection process for many products from automotive parts to electronics.”

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.